Probing exotic magnetic phases in ferrites with neutrons

The decades of the sixties and early seventies saw the neutron diffraction technique playing a major role in the elucidation of magnetic structures in a variety of ferrites in our laboratory. In 1979, Villain in his seminal paper on insulating spin glasses argued that the spinel structure affords topological frustration which can give rise to a variety of perturbed magnetic ordering, depending upon magnetic dilution in the ferrite. The neutron is a unique probe to explore the nature of spatial correlations of magnetic moments in such systems. This paper describes the exciting work carried out at Trombay recently in mixed ferrites which have led to the discovery of exotic magnetic phases like the uniaxial random ferrimagnetic phase and the canted random ferrimagnetic phase involving the coexistence of long-range magnetic order and disorder.     

Apparatus for Neutron Scattering Measurements on Sheared Fluids

We report on the construction of an apparatus to allow neutron scattering measurements on fluids undergoing shear. The apparatus has been used with the cold neutron small-angle-neutron-scattering (SANS) spectrometer at the NIST research reactor and will be made available to users as a permanent part of the NIST facility.     

Materials Research With Neutrons at NIST

The NIST Materials Science and Engineering Laboratory works with industry, standards bodies, universities, and other government laboratories to improve the nation’s measurements and standards infrastructure for materials. An increasingly important component of this effort is carried out at the NIST Center for Neutron Research (NCNR), at present the most productive center of its kind in the United States. This article gives a brief historical account of the growth and activities of the Center with examples of its work in major materials research areas and describes the key role the Center can expect to play in future developments.     

Spin chirality on a two-dimensional frustrated lattice

The collective behaviour of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. We report a study of the spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single-crystal samples of KFe3(OH)6(SO4)2, an ideal Kagomé lattice antiferromagnet. Combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic field induces a transition between states with different non-trivial spin-textures.     

EuFe2As2: Magnetic Structure and Local Charge Distribution Anisotropies as Seen by Resonant X-ray Scattering

Non-resonant and element specific magnetic X-ray scattering has been used to determine the orientation of Eu and Fe magnetic moments in EuFe₂As₂ iron pnictide. Experiments have been carried out on single crystal samples at the ESRF. Resonant measurements on magnetic reflections at the Eu L₃ absorption edge indicate that the orientation of the Eu moments in the antiferromagnetic phase (T<T _N=19 K) lie parallel to the crystallographic a-axis. In addition, non-resonant magnetic X-ray measurements indicate that the Fe moments are aligned along the same direction in the spin density-wave ordered phase (T<T _S=190 K). The temperature dependence of the integrated intensities suggests that the Fe magnetic sublattice is barely affected by the onset of Eu ordering at T _N. The observation of non-zero resonant intensity on nuclear-forbidden reflections with wavevector corresponding to the Fe magnetic propagation vector at both the Eu L₃ and As K absorption edges may be interpreted as the result of the polarization of the Eu 5d and As 4p electronic bands via hybridization with the Fe 3d states.     

Neutron Scattering at Highest Magnetic Fields at the Helmholtz Centre Berlin

The Helmholtz Centre Berlin (HZB), formerly Hahn-Meitner Institute is a user facility for the study of structure and dynamics with neutrons and synchrotron radiation with special emphasis on experiments under extreme conditions. Neutron scattering is uniquely suited to study magnetic properties on a microscopic length scale, because neutrons have comparable wavelengths and, due to their magnetic moment, they interact with the atomic magnetic moments. At HZB a dedicated instrument for neutron scattering at extreme fields is under construction, the Extreme Environment Diffractometer ExED. It is projected according to the “time-of-flight” principle for elastic and inelastic neutron scattering and for the special geometric constraints of analysing samples in a high field magnet. The new magnet will not only allow for novel experiments, it will be at the forefront of development in magnet technology itself. The design of the magnet will follow the Series Connected Hybrid System Technology (SCH) developed at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida. To compromise between the needs of the magnet design for highest fields and the concept of the neutron instrument, the magnetic field will be generated by means of a coned solenoid with horizontal field orientation. By using resistive insert coils, which are mounted in the room temperature bore of a superconducting cable-in-conduit (CIC) magnet, fields above 30 Tesla can be obtained in a geometry optimised for the demands of neutron scattering.     

Detecting the Radiative Decay Mode of the Neutron

Beta decay of the neutron into a proton, electron, and electron antineutrino is occasionally accompanied by the emission of a photon. Despite decades of detailed experimental studies of neutron beta-decay, this rare branch of a fundamental weak decay has never been observed. An experiment to study the radiative beta-decay of the neutron is currently being developed for the NG-6 fundamental physics endstation at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). The experiment will make use of the existing apparatus for the NIST proton-trap lifetime experiment, which can provide substantial background reduction by providing an electron-proton coincidence trigger. Tests and design of a detector for gamma-rays in the 10 keV to 200 keV range are under development. The need for a large solid-angle gamma-ray detector that can operate in a strong magnetic field and at low temperature has led us to consider scintillating crystals in conjunction with avalanche photodiodes. The motivation and experimental technique will be discussed.     

晶场劈裂序参量导致稀土-硼十二化合物中磁性和比热反常的理论研究

在低温条件(T_C≈3~22 K)下, 除了4f电子满壳层的LuB₁₂之外, 其他稀土-硼十二化合物(简称RB₁₂, R为稀土元素Tb, Dy, Ho, Er, Tm)存在磁性和比热的反常现象, 该反常现象的机制至今仍存争议。尽管磁化率曲线上的隆起与R³⁺离子的反铁磁序类似, 但是中子衍射谱和莫斯堡尔谱的实验均没有明确给出稀土离子的磁结构。这些化合物磁化率、熵的反常行为和低温中子谱的结果都与RB₁₂具有幅度调制的磁结构一致。本课题组采用模型和数值计算相结合的方法来探究该反常行为的物理机制。研究表明: 由于稀土离子meV量级的晶场劈裂易受热涨落扰动, 晶场劈裂能不再是一个量子力学参量, 而是一个热力学序参量。物理图像中轨道角动量逐渐淬灭很好地解释了磁性和比热的反常现象。结合该类晶体低温下在(111)晶向的二聚化行为和非弹性散射谱的结果, 对中子散射谱中的双峰结构也给予了一个可能的解释。     

Magnetic Ordering in BaFe$$_{}$$In$$_{0.1}$$0.1O$$_{19}$$19 Hexaferrite

The crystal and magnetic structure by powder neutron diffractometry as well as the magnetic properties by vibration sample magnetometry for the BaFe\(_{11.9}\)In\(_{0.1}\)O\(_{19}\) polycrystalline sample have been performed in a wide temperature range from 10 up to 730 K and in magnetic field up to 14 T. The atomic coordinates and lattice parameters have been Rietveld refined. The Invar effect has been observed in the low-temperature range below 150 K. It was explained by the thermal oscillation anharmonicity of atoms. The increase of the microstress value with decreasing temperature has been defined from Rietveld refinement. It is established that the ferrimagnet–paramagnet phase transition is a standard second-order one. From the macroscopic magnetization measurement, the Curie temperature and ordered magnetic moment per nominal iron ion are obtained. From the microscopic diffraction measurement, the magnetic moments at different atomic position and total magnetic moment per iron ion have been defined at different temperatures. The most likely reasons and the mechanism of magnetic ordering are discussed.     

Ultra-High Resolution Inelastic Neutron Scattering

Two types of ultra high energy resolution neutron scattering instruments, the backscattering spectrometer and the spin echo spectrometer, are described. Examples of the types of research which can be done with these instruments are given and plans for a cold neutron backscattering spectrometer which will be built in the NIST Cold Neutron Research Facility (CNRF) are discussed. It is hoped that this information will be of use to researchers considering neutron scattering experiments at NIST.     

Investigation of Surface Properties of Magnetorheological Elastomers by Atomic Force Microscopy

Magnetorheological elastomers generally consist of a natural or synthetic rubber matrix interspersed with micron-sized ferromagnetic particles. The magneto-elastic properties of such a composite are not merely a sum of the elasticity of the polymer and the stiffness and magnetic properties of the filler, but also the result of a complex synergy of several effects, relevant at different length scales and detectable by different techniques. In our present work we investigate the microstructures, the surface magnetic properties, and the elastic properties of new isotropic and anisotropic magnetorheological elastomers prepared using silicone rubber and soft magnetic carbonyl iron microspheres. Similar samples were previously investigated by means of small-angle neutron scattering, which proved to be a useful method in the investigation of their microscopic properties. We combined the data from the atomic force microscopy measurements with those from small-angle neutron scattering to better understand the complicated behaviour of the studied materials. The measurements were performed by atomic force microscopy in the following modes: standard imaging non-contact atomic force microscopy, magnetic force microscopy, and nanoindentation. A comparative study of samples with different particle concentrations and strength of magnetic field applied during the polymerization process is developed.     

Static and Dynamic Properties of the Insulator–Metal Transition in Magnetic Semiconductors, Including the Perovskites

All magnetic semiconductors experience an insulator–metal transition with decreasing temperature near and below the magnetic ordering temperature. T_c, as long as the carrier concentration, whether electrons or holes, is low enough. For somewhat higher carrier concentrations, a resistivity anomaly ordinarily occurs near T_c, which cannot be explained in terms of scattering from the magnetic fluctuations alone. The thrust of this paper is to review the magnetotransport properties of various magnetic semiconductors and to present arguments that magnetic polarons, a many body state involving charge carriers and the localized magnetic spins in their immediate neighborhoods, are involved in the physical processes leading to the insulator–metal transition. Magnetic polarons have been observed directly by a variety of physical techniques including, for example, magnetic measurements, neutron diffraction, scanning tunneling microscopy, and, most recently, by noise measurements. This article will review these physical manifestations of the existence of magnetic polarons and relate them to the transition.     

Neutron Scattering and Ordering of Nuclear Spins

The ordering of nuclear spins takes place at extremely low temperatures because of the weakness of the interactions between their magnetic moments. Neutron scattering is the traditional and unique method to study the collective ordering of magnetic moments. The combination of the problem (nuclear spin ordering) and the method (neutron scattering) leads to new insight into the many body behaviour of the nuclear spin system and into some of the long standing general questions of the cooperative behaviour of magnetic systems. We describe neutron scattering studies on the nuclear spin systems in Cu and Ag at nano- and subnanokelvin temperatures as an example of a very successful international collaboration based on the work performed at the Low Temperature Laboratory, in the Helsinki University of Technology in Otaniemi. The collaboration was initiated by Olli V. Lounasmaa.     

Neutron Scattering Experiment on Solid 3He

Magnetic neutron scattering probes the order parameter symmetry and magnitude on the atomic length scale, as well as exchange parameters in a critical scattering experiment above the ordering temperature. Therefore we propose an experiment for magnetic neutron diffraction on solid ³ He. Difficulties arise from heat input and small signal intensities, both due to neutron absorption. Thus, a special design of neutron spectrometer, sample cell, and cryostat is required. We present calculated requirements of the involved components. We show the feasibility and possibilities of such an experiment along with first results of performance measurements of some of the parts.     

Test of He-based neutron polarizers at NIST

Neutron spin filters based on polarized ³He are useful over a wide neutron energy range and have a large angular acceptance among other advantages. Two optical pumping methods, spin-exchange and metastability-exchange, can produce the volume of highly polarized ³He gas required for such neutron spin filters. We report a test of polarizers based on each of these two methods on a new cold, monochromatic neutron beam line at the NIST Center for Neutron Research.     

DAVE: A Comprehensive Software Suite for the Reduction,Visualization, and Analysis of Low Energy Neutron Spectroscopic Data

National user facilities such as the NIST Center for Neutron Research (NCNR) require a significant base of software to treat the data produced by their specialized measurement instruments. There is no universally accepted and used data treatment package for the reduction, visualization, and analysis of inelastic neutron scattering data. However, we believe that the software development approach adopted at the NCNR has some key characteristics that have resulted in a successful software package called DAVE (the Data Analysis and Visualization Environment). It is developed using a high level scientific programming language, and it has been widely adopted in the United States and abroad. In this paper we describe the development approach, elements of the DAVE software suite, its usage and impact, and future directions and opportunities for development.     

Magnetic Properties of the Frustrated fcc – Antiferromagnet HoB12 Above and Below T N

We have investigated the magnetic ordering of the frustrated fcc – antiferromagnet HoB₁₂. Below T_N= 7.4 K antiferromagnetic order and a complex phase diagram is observed. Above T_N neutron scattering experiments show strong diffuse scattering. The diffuse signal indicates strong correlations between rare earth moments along the [111] direction well above T_N. The behavior of this component resembles low dimensional magnets which are known to show long range order only at T = 0. Close to T_N correlations perpendicular to the [111] direction get relevant, they diverge toward T_N. Thus we observe a complex ordering process where the frustration is lifted in steps. The experimental data and their interpretation are presented, some of the possible microscopic origins are discussed.      

NIST Calibration of a Neutron Spectrometer ROSPEC

A neutron spectrometer was acquired for use in the measurement of National Institute of Standards and Technology neutron fields. The spectrometer included options for the measurement of low and high energy neutrons, for a total measurement range from 0.01 eV up to 17 MeV. The spectrometer was evaluated in calibration fields and was used to determine the neutron spectrum of an Americium-Beryllium neutron source. The calibration fields used included bare and moderated ²⁵²Cf, monoenergetic neutron fields of 2.5 MeV and 14 MeV, and a thermal-neutron beam. Using the calibration values determined in this exercise, the spectrometer gives a good approximation of the neutron spectrum, and excellent values for neutron fluence, for all NIST calibration fields. The spectrometer also measured an Americium-Beryllium neutron field in a NIST exposure facility and determined the field quite well. The spectrometer measured scattering effects in neutron spectra which previously could be determined only by calculation or integral measurements.     

Magnetic Field Stabilization for Magnetically Shielded Volumes by External Field Coils

For highly sensitive magnetic measurements, e.g., a measurement of the neutron electric dipole moment (EDM), the magnetic field has to be stable in time on a level below picoTesla. One of several measures we employ to achieve this uses an external field coil system which can stabilize the ambient external field at a predefined value. Here we report on the construction and characterization of such a system in the magnetic test facility at PSI. The system actively stabilizes the field along the axis of the EDM experiment by means of four coils in a Helmholtz-like configuration. Additional coils serve to compensate for transverse ambient field components. Because of the long integration times in the EDM experiment (about 100 s or more) only slow disturbances have to be corrected for. The performance of the system has been measured using static and moving magnetic sources and suppression factors in excess of 200 have been observed.